One of Jupiter's moons is thought to have conditions that are 'ripe for life'.

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Ian O'Neill is the Space Science Producer at Discovery News, and founder and editor of Astroengine.

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Rutland, United Kingdom - Jupiter: the largest planet in the solar system, 'protector' of the terrestrial planets, host to 66 moons and, potentially, home to life. The Jovian system is therefore one of the most intriguing and enigmatic targets for future space missions. And now, nine years after our last robotic foray to Jupiter, NASA has a mission powering its way through interplanetary space.On May 10, NASA's Juno spacecraft took a photograph of the Big Dipper (or "The Plough") to test its JunoCam instrument. In four-and-a-half years time, that camera's vista will be filled with Jupiter and its task is to study the gas giant's atmosphere, so it's good to make sure everything is in working order ahead of orbital insertion. But to study Jupiter's

Los Angeles, CA - Jupiter: the largest planet in the solar system, "protector" of the terrestrial planets, host to 66 moons and, potentially, home to life. The Jovian system is therefore one of the most intriguing and enigmatic targets for future space missions. And now, nine years after our most recent robotic foray to Jupiter, NASA has a mission powering its way through interplanetary space.

On May 10, NASA's Juno spacecraft took a photograph of the Big Dipper (or "The Plough") to test its JunoCam instrument. In four-and-a-half years time, that camera's vista will be filled with Jupiter, and its task is to study the gas giant's atmosphere, so it's good to make sure everything is in working order ahead of orbital insertion. But to study Jupiter's atmosphere is just one of the mission objectives of the solar-powered spacecraft.

The plan is to see Juno orbit Jupiter 33 times, so it can use its nine sophisticated instruments to probe deep beneath its atmosphere to glean information about the planet's origin, structure and magnetosphere. The mission - part of NASA's New Frontiers Program - will also examine the nature of Jupiter's (potentially) solid core and will search for clues as to how the gas giant formed, which, in turn, will help us to understand the origins of the Solar System. Juno will have a polar orbit, meaning it will fly over the planet's poles, rather than around its equator. It will therefore get some pretty spectacular views, particularly of the planet's aurorae, while spending the least amount of time as possible inside Jupiter's damaging radiation belts.

"Jupiter, it is believed, has played a large part in gravitationally 'vacuuming' errant space rocks from careening into our planet, sanitising it of all life."

Studying Jupiter also helps us understand why the Solar System has a comparatively serene inner system of planets. Although - as the cratered scars on the Moon will attest - our planet (plus Mercury, Venus and Mars) underwent a savage barrage of comets and asteroids in the Solar System's early history. The Late Heavy Bombardment ravaged the terrestrial planets around four billion years ago and they have since undergone intermittent periods of impact activity.

But since life started to form on Earth over the past four billion years, a long period of calm has allowed life to evolve from single-celled microorganisms to the thriving ecosystem we know today. Jupiter, it is believed, has played a large part in gravitationally "vacuuming" errant space rocks from careening into our planet, sanitising the giant of all life.

It is interesting to note, however, that Jupiter also has a habit of modifying the orbits of comets, sending them our way. Jupiter therefore turns from perceived protector to a cosmic killer. But, according to Dave Waltham, head of the earth sciences department at Royal Holloway, University of London, mass extinctions caused by huge chunks of rock raining down on Earth has an upside:

"What you can definitely say is that they cause some mass extinctions. Lots of people argue, however, that impacts can be good because they stir things up and stop the biosphere from becoming stuck in a rut. Certainly for human beings, we wouldn't be here if the dinosaurs hadn't been wiped out."

JUICE is next

So, in an effort to better understand Jupiter and planetary formation theories, Juno was launched on August 5, 2011, and has since travelled beyond the orbit of Mars. But, as announced by the European Space Agency earlier this month, another Jupiter mission is hot on Juno's tail. The Jupiter Icy moons Explorer - aka "JUICE" - will be ESA's first large-class mission chosen as part of the agency's Cosmic Vision 2015-2025 programme. It is expected to cost 870 million euros (US $1.1bn), will be launched in 2022 and will reach Jupiter in 2030.

Both Juno and JUICE are a departure from their predecessor, Galileo - NASA's hugely successful mission that ended in 2003. Galileo was powered by two radioisotope thermoelectric generators (RTGs) containing pellets of radioactive plutonium-238 that generated heat that could be converted into electricity. At the distance of Jupiter's orbit - with a semi-major axis of 5.2 AU, or more than five-times the distance between the Sun and the Earth - the Sun's energy was far too weak for the solar panels of the time to be practical. However, during the subsequent years, advances in solar cell technology can allow modern space missions to utilise the faint sunlight for power production. Juno became the first spacecraft to venture beyond the orbit of Mars, equipped with solar panels that could utilise the sunlight Jupiter receives - a mere four per cent that of what the Earth receives.

Juno's solar panel array, however, is huge. The three solar panel sections are arranged in a Y-shaped configuration and span an area of 60 square metres (650 sq ft) - the largest array ever used on a robotic NASA mission. The JUICE spacecraft is expected to also boast a solar-collecting array totaling 60-70 square metres (650-750 sq ft), although its design has yet to be finalised.

Contamination danger

During the Galileo mission, great care had to be taken to dispose of the spacecraft; simply assigning the 2.6 tonne spacecraft to a Jovian "graveyard orbit" would have been considered reckless. The concern wasn't for trashing the Jovian orbit with debris that may become a collision issue for future spacecraft cruising around the gas giant (a problem that is of growing concern for low-Earth orbit); the concern was for the bacteria that may have hitchhiked on the spacecraft's frame and small quantities of radioactive material Galileo was carrying in its RTGs. Why should this be a problem for the Jovian system? Well, there may be life hiding on one (or more) of Jupiter's moons.

The risk of contaminating this hypothetical life was considered too high, so mission managers used the remnants of Galileo's fuel to send it on a death-dive into the gas giant's thick atmosphere to avoid a future collision with one of the Jovian moons. The spacecraft was quickly incinerated during atmospheric entry on September 21, 2003.

"The most interesting moon of the trio is Europa, a world that is long thought to host the conditions ripe for life."

As can be noted from the mission's slightly contrived acronym, JUICE isn't primarily going after the gas giant itself (although it will carry out observations of its atmosphere and magnetosphere); it is in fact going to explore the Jovian satellites, so, once its mission is complete, it is likely JUICE (and Juno) will face the same fate as Galileo.

But what is this hypothetical alien life we are trying to protect? And how can JUICE help us find it?

Complex lifeforms?

It is thought that Jupiter's largest moons, Europa, Ganymede and Callisto, have extensive sub-surface oceans that may make ideal habitats for life. The most interesting moon of the trio is Europa, a world that is long thought to host the conditions ripe for life. Its cracked, icy crust is surprisingly smooth - it has the reputation for looking like a "cue ball" - and there is little evidence of impact craters. This suggests the ice is in a state of circulation; internal heating of the moon keeps a subsurface ocean in a liquid state, cycling it toward the surface, replenishing the surface ice through the cracks.

Planetary scientist and Europa specialist Richard Greenburg, of the University of Arizona, came to the conclusion in 2009 that the 1,569 kilometre-wide moon's ice dynamics may allow oxygen and nutrients to cycle into the sub-surface ocean. Where there's water, simple microorganisms may thrive; where there's a highly oxygenated sub-surface ocean, more complex creatures may have evolved. In a Discovery News interview, Greenburg said: "If there is evidence for oxygenation, then the presence of oxygen would certainly increase the environmental conditions for life. It doesn't mean life is there, just that there might be free oxygen available to support biological processes."

So, during its mission, ESA plans to send JUICE to Callisto, the Solar System's most cratered body. It will then carry out two passes of Europa to analyse its icy surface. Then, by 2032, it will enter Ganymede's orbit, so it can study that moon's surface and sub-surface oceans. However, the Europa flybys are likely to return some of the most exciting results - the spacecraft will gauge the thickness of its icy crust and identify candidate sites for future lander missions that may incorporate a submersible vehicle to melt through the ice or find a crack in the crust.

Although it is highly speculative, Europa may nurture the ideal conditions for life. Not only is there likely an extensive sub-surface ocean, it may be heated by volcanic vents - much like the vents that are found along fault lines under Earth's oceans. It is well known that, although the conditions are extreme, hardy microorganisms known as "extremophiles" thrive around these vents, using the heat as a source of energy. Also, any biology would be naturally protected by the icy crust, perhaps mitigating the worst effects of space weather radiation.

If prebiotic compounds are present on Europa, then perhaps basic biology has been sparked in the depths of the protected sub-surface ocean. Now throw in the oxygenated ocean hypothesis, and there may be an opportunity for Europan life to get a foothold. Could multicellular organisms - perhaps as complex as terrestrial jellyfish - be thriving on this moon?

Sadly, although JUICE will give us some clues about Europa's surface and sub-surface composition, until we land a surface mission on the moon's ice in the future, we may never find out.

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